migration: deprecate != 'online' migrations

Since long time the only supported method for migration
is actually 'online'. In the API schema, the methods
are actually an enumeration with a single value ('online').

This patch adds an explicit deprecation reminder.

Change-Id: I40a9c9ab26d0bb4dd9e3a2ff449b318fa0e96f49
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29555
Reviewed-by: Dan Kenigsberg <danken@redhat.com>